Ink jet heads designed to eject ink by using shear mode of piezoelectric material have conventionally been proposed (refer to paragraph 0018 and FIG. 4 of Japanese Laid-open Patent Application No. H04-259563, for example). This kind of ink jet head includes an actuator member 100 having a plurality of grooves formed in an element of piezoelectric material which is formed by bonding pieces of the piezoelectric material of which piezoelectric properties have been oriented along a thickness direction such that the pieces of the piezoelectric material have opposite poling directions, a cover member 110 in which an ink supply opening 111 and a common ink chamber 112 are formed, and a nozzle plate 120 in which nozzle holes 121 are formed. As the actuator member 100, the cover member 110 and the nozzle plate 120 are bonded together, there are formed ink chambers 112 by the multiple grooves as shown in FIG. 13, for example. Inside the individual ink chambers 112, there are formed electrodes 101 for applying electric fields on walls separating the ink chambers.
A bottom surface of a rear half part 102 of the ink chamber 112 is machined into a curved shape. In the ink chamber 112, there is further formed a flat part 103 serving as an electrode extension part for achieving connection to an external circuit. The ink jet head and a driving IC 130 placed on a supporting substrate 140 are electrically connected by wire bonding technology by means of electrodes 104 formed on the flat parts 103 and aluminum wires 131. Alternatively, the ink jet head can be connected to a flexible board to which the driving IC 130 is connected by ACF (anisotropic conductive film) bonding technology, for example.
Next, a method of extending the electrodes in the individual ink chambers of the conventional ink jet head to the flat parts is explained with reference to FIGS. 14A and 14B. First, a dry film resist 150 is laminated and hardened on a primary surface of the actuator member 100. Next, grooves which will later become the ink chambers are formed by dicing the piezoelectric material to a depth of about half the thickness thereof by using a dicing blade 160 of a dicer. Then, the curved shape of the rear half part 102 of each ink chamber corresponding to the diameter of the dicing blade is formed by raising the dicing blade 160. Subsequently, only the dry film resist 150 is cut away at the flat part.
After an array of ink chambers has been formed in the aforementioned fashion, metal electrode material such as aluminum (Al) or copper (Cu) is deposited in each of the ink chambers by using sputtering or plating technology as shown in FIG. 14B. A metal film is also deposited in a similar way at openings in the dry film resist 150 formed at the curved bottom surface of the rear half part 102 and the flat part of each ink chamber to form electrodes for connection to the external circuit.
The actuator member 100 thus formed is driven in the shear mode by applying reverse-phased voltages to electrodes formed on opposite sides of each wall separating the ink chambers. Specifically, the walls separating the ink chambers deform in the shape of “<” at a joint surface of the ink chamber walls where the pieces of the piezoelectric material are bonded such that the poling directions of the piezoelectric pieces are symmetrically arranged, so that the volumetric capacity of each of the ink chambers varies. A variation in the volumetric capacity of the ink chamber results in a change in pressure applied to the ink in the ink chamber, whereby ink droplets are ejected through a fine nozzle located at a far end of the ink chamber.
In the aforementioned structure of the conventional ink jet head, only a front end section (front half portion) situated at the front of the ink supply opening 111 and the common ink chamber 112 constitutes a so-called active area which contributes to producing ink ejecting action, while a rear end section (rear half portion) including the ink supply opening 111 constitutes an area for supplying the ink. Further, the large-sized curved parts and flat parts 103 are used as extension electrodes for connecting the internal electrodes 101 of the individual ink chambers to the external circuit. In other words, these flat parts 103 are areas for establishing electrical connections to the electrodes which are connected to the driving IC 130.
In this structure of the ink jet head, portions other than the active area which contributes to the ink ejecting action are extremely large. This increases materials cost and, therefore, there has been a problem that the ink jet head could not be manufactured at low cost.
In addition, as it is necessary to extend the electrode 101 in each ink chamber up to the flat part 103 on the piezoelectric material like PZT having a high dielectric constant, the ink jet head will have a large capacitance. For this reason, a drive pulse waveform applied for driving the actuator becomes unsharp, leading to a problem that it becomes difficult to achieve high-speed printing by driving the actuator at high speed.
Although it is possible to improve unsharpness of the applied driving waveform by increasing an applied voltage, the amount of heat generated by driving the actuator increases and the temperature of the actuator increases when the applied voltage is increased. Consequently, the viscosity of the ink changes, leading to a problem that it is impossible to perform high-precision printing in a stable manner. This approach would also develop such problems that a driving IC to which a high voltage can be applied is costly, durability is affected by early deterioration of properties of the piezoelectric material caused by application of the high voltage, and it is difficult to reduce power consumption.
Under these circumstances, a film having a low dielectric constant is preformed between the piezoelectric material and the electrodes in the portions other than the active area of the internal electrodes 101 of the individual ink chambers so that the capacitance produced by the portions other than the active area is lowered to an almost negligible level. However, an extremely expensive ECR-CVD system is needed to form an Si—N film having a low dielectric constant, for instance, by a low-temperature process on PZT which is a piezoelectric material having such a low Curie point as approximately 200° C. Thus, there has been a problem that inexpensive ink jet heads could not be manufactured due to an increase in manufacturing cost.
To cope with the foregoing problems, there is proposed a structure in which an area for extending an ink supply opening and internal electrodes of individual ink chambers is taken in a longitudinal direction of piezoelectric material as shown in FIG. 15, for example (refer to paragraph 0008 and FIG. 1 of Japanese Laid-open Patent Application No. H09-094954, for example). According to this proposal, the ink supply opening for supplying ink is provided in a rear end section of an active area of the piezoelectric material and an electrode 101 inside each ink chamber 112 is extended along a side surface on an ink supply side or along a side surface on an ink ejecting side to establish electrical connections to electrodes 171 which are connected to a driving IC 170.
Since portions other than the active area of actuators 100 are so little that a reduction in materials cost can be achieved with respect to the piezoelectric material. However, the electrodes 101 inside the ink chambers 112 must be bent generally at right angles along side surfaces of the actuators to extend the electrodes outward. To achieve this, it is necessary to structure the individual actuators 100 in small pieces and form a metal film on the side surface of each actuator to establish an electrical connection to the electrode 101 of each ink chamber 112. Such a metal film forming process is extremely inefficient.
Furthermore, to separate the individual electrodes, it is necessary to carry out beforehand resist patterning or an electrode separation process by using a dicing technique or a YAG laser after forming an outward-extending unseparated electrode. This approach has a problem that it involves an extremely complex process which results in low productivity, a low manufacturing yield and an increase in manufacturing cost. In addition, there is a high potential that the electrodes extended outward will break in a later process at bent parts from where the electrodes are extended from the ink chambers 112 to the side surfaces of the individual actuators. Thus, this structure has had such problems as deterioration of manufacturing yield and low environmental reliability.
Aiming at a solution of these problems, the present Applicant has proposed an ink jet head in which electrodes for providing external connections are formed of electrically conductive material filled into ink chambers (refer to paragraphs 0067 to 0072 and FIG. 1 of Japanese Laid-open Patent Application No. 2002-178518, for example). FIG. 16 (FIGS. 16A-16C) illustrates the ink jet head, in which FIG. 16A is a cross-sectional front view, FIG. 16B is a cross-sectional view taken along a line X-X, and FIG. 16C is a cross-sectional view taken along a line Y-Y.
According to this proposal, the electrodes for establishing external connections are formed of electrically conductive material 105 filled into the ink chambers as shown in FIG. 16, so that it is made unnecessary to extend the internal electrodes of the ink chambers to the outside thereof unlike the case of the prior art. Since actuators 100 necessitate almost no portions other than an active area, it is possible to achieve a reduction in materials cost.
As the capacitance decreases, it becomes possible to increase driving frequency and thereby perform high-speed printing. Furthermore, since driving voltage can be decreased, it becomes possible to lower withstand voltage of a driving IC, making it possible to achieve a cost reduction of the driving IC and a reduction in power consumption for driving the same.
Due to demands for a further reduction in materials cost and higher-speed printing, however, there has been a move toward narrow-pitch design of ink jet heads in which individual ink chambers of an ink chamber array are arranged at smaller intervals. As a result of this move, the areas of the electrodes for establishing electrical connections to an external circuit decrease in the structure shown in FIG. 16.
For this reason, large increases or variations in connecting resistance could occur at electrical connections between the ink jet head and the external circuit, such as a flexible board, causing a problem that environmental reliability would deteriorate. Also, a driving waveform applied for driving the actuator becomes unsharp, leading to a problem that it becomes difficult to achieve high-speed printing by driving the actuator at high speed.
This invention has been made in consideration of the aforementioned situation. Accordingly, it is an object of the invention to provide a low-cost ink jet head which can be connected to an external circuit with low connecting resistance in a stable fashion, the ink jet head having high environmental reliability and a capability to perform high-speed printing.